Fix issues found running tests
//mobile/flutter/tests/app:basic_runner_test_*
during a google3 roll.

The following exception was being thrown

_TypeError: type 'String' is not a subtype of type 'File'

```
8)
<asynchronous suspension>
utter_command.dart:347:18)
<asynchronous suspension>
/flutter_command.dart:282:33)
<asynchronous suspension>
xt.dart:142:29)
<asynchronous suspension>

packages/flutter_tools/lib/src/commands/run.dart

diff --git a/packages/flutter_tools/lib/src/commands/run.dart b/packages/flutter_tools/lib/src/commands/run.dart
index 730be16..f1df87b 100644
--- a/packages/flutter_tools/lib/src/commands/run.dart
+++ b/packages/flutter_tools/lib/src/commands/run.dart
@@ -364,13 +364,16 @@
     }).toList();
 
     ResidentRunner runner;
+    final String applicationBinaryPath = argResults['use-application-binary'];
     if (hotMode) {
       runner = new HotRunner(
         flutterDevices,
         target: targetFile,
         debuggingOptions: _createDebuggingOptions(),
         benchmarkMode: argResults['benchmark'],
-        applicationBinary: argResults['use-application-binary'],
+        applicationBinary: applicationBinaryPath == null
+            ? null
+            : fs.file(applicationBinaryPath),
         projectRootPath: argResults['project-root'],
         packagesFilePath: globalResults['packages'],
         dillOutputPath: argResults['output-dill'],
@@ -383,7 +386,9 @@
         target: targetFile,
         debuggingOptions: _createDebuggingOptions(),
         traceStartup: traceStartup,
-        applicationBinary: argResults['use-application-binary'],
+        applicationBinary: applicationBinaryPath == null
+            ? null
+            : fs.file(applicationBinaryPath),
         stayResident: stayResident,
         ipv6: ipv6,
       );