Enable the resource shrinker (#40610)

dev/bots/test.dart

diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index eb03075..823956f 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -1018,6 +1018,7 @@
   if (subShard == 'gradle1') {
     await _runDevicelabTest('gradle_plugin_light_apk_test', env: env);
     await _runDevicelabTest('gradle_plugin_fat_apk_test', env: env);
+    await _runDevicelabTest('gradle_r8_test', env: env);
   }
   if (subShard == 'gradle2') {
     await _runDevicelabTest('gradle_plugin_bundle_test', env: env);

dev/devicelab/bin/tasks/gradle_r8_test.dart

diff --git a/dev/devicelab/bin/tasks/gradle_r8_test.dart b/dev/devicelab/bin/tasks/gradle_r8_test.dart
new file mode 100644
index 0000000..49017e7
--- /dev/null
+++ b/dev/devicelab/bin/tasks/gradle_r8_test.dart
@@ -0,0 +1,77 @@
+// Copyright (c) 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/framework/utils.dart';
+import 'package:path/path.dart' as path;
+
+/// Tests that flutter build apk uses R8 by default by adding an
+/// invalid proguard rule and evaluating the error message.
+Future<void> main() async {
+  await task(() async {
+
+    section('Find Java');
+
+    final String javaHome = await findJavaHome();
+    if (javaHome == null)
+      return TaskResult.failure('Could not find Java');
+    print('\nUsing JAVA_HOME=$javaHome');
+
+    section('Create Flutter app project');
+
+    final Directory tempDir = Directory.systemTemp.createTempSync('flutter_module_test.');
+    final Directory projectDir = Directory(path.join(tempDir.path, 'hello'));
+    try {
+      await inDirectory(tempDir, () async {
+        await flutter(
+          'create',
+          options: <String>[
+            '--org', 'io.flutter.devicelab',
+            'hello',
+          ],
+        );
+      });
+
+      section('Add incorrect proguard rules');
+
+      final File proguardRules = File(path.join(
+        projectDir.path,
+        'android',
+        'app',
+        'proguard-rules.pro',
+      ));
+      proguardRules.writeAsStringSync('invalidRule', flush: true);
+
+      section('Build release APK');
+
+      final StringBuffer stderr = StringBuffer();
+
+      await inDirectory(projectDir, () async {
+        await evalFlutter(
+          'build',
+          options: <String>[
+            'apk',
+            '--target-platform', 'android-arm',
+            '--verbose',
+          ],
+          canFail: true,
+          stderr: stderr,
+        );
+      });
+
+      if (!stderr.toString().contains('com.android.tools.r8.CompilationFailedException')) {
+        return TaskResult.failure('Expected com.android.tools.r8.CompilationFailedException in stderr.');
+      }
+
+      return TaskResult.success(null);
+    } catch (e) {
+      return TaskResult.failure(e.toString());
+    } finally {
+      rmTree(tempDir);
+    }
+  });
+}

packages/flutter_tools/gradle/flutter.gradle

diff --git a/packages/flutter_tools/gradle/flutter.gradle b/packages/flutter_tools/gradle/flutter.gradle
index 4d65284..c3328d7 100644
--- a/packages/flutter_tools/gradle/flutter.gradle
+++ b/packages/flutter_tools/gradle/flutter.gradle
@@ -154,7 +154,7 @@
             }
         }
 
-        if (shouldUseProguard(project)) {
+        if (shouldShrinkResources(project)) {
             String flutterProguardRules = Paths.get(flutterRoot.absolutePath, "packages", "flutter_tools",
                     "gradle", "flutter_proguard_rules.pro")
             project.android.buildTypes {
@@ -164,7 +164,8 @@
                     minifyEnabled true
                     // Enables resource shrinking, which is performed by the
                     // Android Gradle plugin.
-                    shrinkResources true
+                    // NOTE: The resource shrinker can't be used for libraries.
+                    shrinkResources isBuiltAsApp(project)
                     // Fallback to `android/app/proguard-rules.pro`.
                     // This way, custom Proguard rules can be configured as needed.
                     proguardFiles project.android.getDefaultProguardFile("proguard-android.txt"), flutterProguardRules, "proguard-rules.pro"
@@ -393,13 +394,19 @@
         return false
     }
 
-    private static Boolean shouldUseProguard(Project project) {
-        if (project.hasProperty('proguard')) {
-            return project.property('proguard').toBoolean()
+    private static Boolean shouldShrinkResources(Project project) {
+        if (project.hasProperty("shrink")) {
+            return project.property("shrink").toBoolean()
         }
         return false
     }
 
+    private static Boolean isBuiltAsApp(Project project) {
+        // Projects are built as applications when the they use the `com.android.application`
+        // plugin.
+        return project.plugins.hasPlugin("com.android.application");
+    }
+
     private static Boolean buildPluginAsAar() {
         return System.getProperty('build-plugins-as-aars') == 'true'
     }