Eliminate Dart 1 support from Flutter test (#21408)

packages/flutter_tools/bin/fuchsia_tester.dart

@@ -121,7 +121,6 @@ Future<Null> run(List<String> args) async {
       watcher: collector,
       ipv6: false,
       enableObservatory: collector != null,
-      previewDart2: true,
       precompiledDillPath: dillFile.path,
       concurrency: math.max(1, platform.numberOfProcessors - 2),
     );

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

@@ -167,7 +167,6 @@ class TestCommand extends FlutterCommand {
       startPaused: startPaused,
       ipv6: argResults['ipv6'],
       machine: machine,
-      previewDart2: true,
       trackWidgetCreation: argResults['track-widget-creation'],
       updateGoldens: argResults['update-goldens'],
       concurrency: jobs,

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -68,7 +68,6 @@ void installHook({
   bool enableObservatory = false,
   bool machine = false,
   bool startPaused = false,
-  bool previewDart2 = true,
   int port = 0,
   String precompiledDillPath,
   bool trackWidgetCreation = false,
@@ -88,7 +87,6 @@ void installHook({
       startPaused: startPaused,
       explicitObservatoryPort: observatoryPort,
       host: _kHosts[serverType],
-      previewDart2: previewDart2,
       port: port,
       precompiledDillPath: precompiledDillPath,
       trackWidgetCreation: trackWidgetCreation,
@@ -339,7 +337,6 @@ class _FlutterPlatform extends PlatformPlugin {
     this.startPaused,
     this.explicitObservatoryPort,
     this.host,
-    this.previewDart2,
     this.port,
     this.precompiledDillPath,
     this.trackWidgetCreation,
@@ -354,7 +351,6 @@ class _FlutterPlatform extends PlatformPlugin {
   final bool startPaused;
   final int explicitObservatoryPort;
   final InternetAddress host;
-  final bool previewDart2;
   final int port;
   final String precompiledDillPath;
   final bool trackWidgetCreation;
@@ -447,15 +443,13 @@ class _FlutterPlatform extends PlatformPlugin {
 
       printTrace('test $ourTestCount: starting shell process');
 
-      // [precompiledDillPath] can be set only if [previewDart2] is [true].
-      assert(precompiledDillPath == null || previewDart2);
       // If a kernel file is given, then use that to launch the test.
       // Otherwise create a "listener" dart that invokes actual test.
       String mainDart = precompiledDillPath != null
           ? precompiledDillPath
           : _createListenerDart(finalizers, ourTestCount, testPath, server);
 
-      if (previewDart2 && precompiledDillPath == null) {
+      if (precompiledDillPath == null) {
         // Lazily instantiate compiler so it is built only if it is actually used.
         compiler ??= new _Compiler(trackWidgetCreation, projectRootDirectory);
         mainDart = await compiler.compile(mainDart);
@@ -705,10 +699,6 @@ class _FlutterPlatform extends PlatformPlugin {
   }
 
   String _getBundlePath(List<_Finalizer> finalizers, int ourTestCount) {
-    if (!previewDart2) {
-      return null;
-    }
-
     if (precompiledDillPath != null) {
       return artifacts.getArtifactPath(Artifact.flutterPatchedSdkPath);
     }

packages/flutter_tools/lib/src/test/runner.dart

@@ -4,7 +4,6 @@
 
 import 'dart:async';
 
-import 'package:args/command_runner.dart';
 import 'package:meta/meta.dart';
 import 'package:test/src/executable.dart' as test; // ignore: implementation_imports
 
@@ -29,20 +28,12 @@ Future<int> runTests(
   bool startPaused = false,
   bool ipv6 = false,
   bool machine = false,
-  bool previewDart2 = true,
   String precompiledDillPath,
   bool trackWidgetCreation = false,
   bool updateGoldens = false,
   TestWatcher watcher,
   @required int concurrency,
 }) async {
-  if (trackWidgetCreation && !previewDart2) {
-    throw new UsageException(
-      '--track-widget-creation is valid only when previewDart2 is specified.',
-      null,
-    );
-  }
-
   // Compute the command-line arguments for package:test.
   final List<String> testArgs = <String>[];
   if (!terminal.supportsColor) {
@@ -83,7 +74,6 @@ Future<int> runTests(
     machine: machine,
     startPaused: startPaused,
     serverType: serverType,
-    previewDart2: previewDart2,
     precompiledDillPath: precompiledDillPath,
     trackWidgetCreation: trackWidgetCreation,
     updateGoldens: updateGoldens,