Simplify iOS debug build (#17145)

iOS debug builds always run in interpreted mode whether on device or on
simulator. In both cases, we can skip snapshotting and link against an
empty App.framework. Previously, we did this for iOS simulator builds.
This does the same for device builds.

Previously, debug iOS builds used gen_snapshot to generate a core
snapshot, then used 'xxd' to generate C files containing the snapshot
data in buffers named kDartVmSnapshotData and kDartIsolateSnapshotData,
which are then compiled/linked into App.framework. This is unnecessary
since the VM compiled into Flutter.framework already contains this data.

packages/flutter_tools/test/base/build_test.dart

diff --git a/packages/flutter_tools/test/base/build_test.dart b/packages/flutter_tools/test/base/build_test.dart
index e58c87f..4c286ea 100644
--- a/packages/flutter_tools/test/base/build_test.dart
+++ b/packages/flutter_tools/test/base/build_test.dart
@@ -22,7 +22,6 @@
 class MockFlutterVersion extends Mock implements FlutterVersion {}
 class MockArtifacts extends Mock implements Artifacts {}
 class MockXcode extends Mock implements Xcode {}
-class MockXxd extends Mock implements Xxd {}
 
 class _FakeGenSnapshot implements GenSnapshot {
   _FakeGenSnapshot({
@@ -586,7 +585,6 @@
     Snapshotter snapshotter;
     MockArtifacts mockArtifacts;
     MockXcode mockXcode;
-    MockXxd mockXxd;
 
     setUp(() async {
       fs = new MemoryFileSystem();
@@ -608,7 +606,6 @@
       snapshotter = new Snapshotter();
       mockArtifacts = new MockArtifacts();
       mockXcode = new MockXcode();
-      mockXxd = new MockXxd();
       for (BuildMode mode in BuildMode.values) {
         when(mockArtifacts.getArtifactPath(Artifact.dartVmEntryPointsTxt, TargetPlatform.ios, mode)).thenReturn(kVmEntrypoints);
         when(mockArtifacts.getArtifactPath(Artifact.dartIoEntriesTxt, TargetPlatform.ios, mode)).thenReturn(kIoEntries);
@@ -623,25 +620,11 @@
       FileSystem: () => fs,
       GenSnapshot: () => genSnapshot,
       Xcode: () => mockXcode,
-      Xxd: () => mockXxd,
     };
 
-    testUsingContext('builds iOS debug AOT snapshot', () async {
-      fs.file('main.dill').writeAsStringSync('binary magic');
-
+    testUsingContext('iOS debug AOT snapshot is invalid', () async {
       final String outputPath = fs.path.join('build', 'foo');
-      fs.directory(outputPath).createSync(recursive: true);
-
-      genSnapshot.outputs = <String, String>{
-        fs.path.join(outputPath, 'vm_snapshot_data'): '',
-        fs.path.join(outputPath, 'vm_snapshot_instr'): '',
-        fs.path.join(outputPath, 'isolate_snapshot_data'): '',
-        fs.path.join(outputPath, 'isolate_snapshot_instr'): '',
-        fs.path.join(outputPath, 'snapshot.d'): '',
-        fs.path.join(outputPath, 'snapshot_assembly.S'): '',
-      };
-
-      final int genSnapshotExitCode = await snapshotter.buildAotSnapshot(
+      expect(await snapshotter.buildAotSnapshot(
         platform: TargetPlatform.ios,
         buildMode: BuildMode.debug,
         mainPath: 'main.dill',
@@ -649,26 +632,7 @@
         outputPath: outputPath,
         preferSharedLibrary: false,
         previewDart2: true,
-      );
-
-      expect(genSnapshotExitCode, 0);
-      expect(genSnapshot.callCount, 1);
-      expect(genSnapshot.snapshotType.platform, TargetPlatform.ios);
-      expect(genSnapshot.snapshotType.mode, BuildMode.debug);
-      expect(genSnapshot.packagesPath, '.packages');
-      expect(genSnapshot.additionalArgs, <String>[
-        '--vm_snapshot_data=${fs.path.join(outputPath, 'vm_snapshot_data')}',
-        '--isolate_snapshot_data=${fs.path.join(outputPath, 'isolate_snapshot_data')}',
-        '--url_mapping=dart:ui,${fs.path.join(skyEnginePath, 'lib', 'ui', 'ui.dart')}',
-        '--url_mapping=dart:vmservice_io,${fs.path.join(skyEnginePath, 'sdk_ext', 'vmservice_io.dart')}',
-        '--dependencies=${fs.path.join(outputPath, 'snapshot.d')}',
-        '--reify-generic-functions',
-        '--strong',
-        '--no-checked',
-        '--conditional_directives',
-        '--snapshot_kind=core',
-        'snapshot.dart',
-      ]);
+      ), isNot(equals(0)));
     }, overrides: contextOverrides);
 
     testUsingContext('builds iOS profile AOT snapshot', () async {
@@ -702,13 +666,13 @@
         '--isolate_snapshot_data=${fs.path.join(outputPath, 'isolate_snapshot_data')}',
         '--url_mapping=dart:ui,${fs.path.join(skyEnginePath, 'lib', 'ui', 'ui.dart')}',
         '--url_mapping=dart:vmservice_io,${fs.path.join(skyEnginePath, 'sdk_ext', 'vmservice_io.dart')}',
+        '--embedder_entry_points_manifest=$kVmEntrypoints',
+        '--embedder_entry_points_manifest=$kIoEntries',
         '--dependencies=${fs.path.join(outputPath, 'snapshot.d')}',
         '--reify-generic-functions',
         '--strong',
         '--no-checked',
         '--conditional_directives',
-        '--embedder_entry_points_manifest=$kVmEntrypoints',
-        '--embedder_entry_points_manifest=$kIoEntries',
         '--snapshot_kind=app-aot-assembly',
         '--assembly=${fs.path.join(outputPath, 'snapshot_assembly.S')}',
         'main.dill',
@@ -746,11 +710,11 @@
         '--isolate_snapshot_data=${fs.path.join(outputPath, 'isolate_snapshot_data')}',
         '--url_mapping=dart:ui,${fs.path.join(skyEnginePath, 'lib', 'ui', 'ui.dart')}',
         '--url_mapping=dart:vmservice_io,${fs.path.join(skyEnginePath, 'sdk_ext', 'vmservice_io.dart')}',
+        '--embedder_entry_points_manifest=$kVmEntrypoints',
+        '--embedder_entry_points_manifest=$kIoEntries',
         '--dependencies=${fs.path.join(outputPath, 'snapshot.d')}',
         '--reify-generic-functions',
         '--strong',
-        '--embedder_entry_points_manifest=$kVmEntrypoints',
-        '--embedder_entry_points_manifest=$kIoEntries',
         '--snapshot_kind=app-aot-assembly',
         '--assembly=${fs.path.join(outputPath, 'snapshot_assembly.S')}',
         'main.dill',