Make support for async/await syntax optional in the frontend.

pkg/compiler/lib/src/resolution/members.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.resolution.members;
 
 import '../common.dart';
 import '../common/names.dart' show Selectors;
 import '../common/resolution.dart' show Feature;
 import '../compiler.dart' show Compiler;
@@ skipping 3518 matching lines @@
       } else if (!node.isArrowBody && currentAsyncMarker.isYielding) {
         reporter.reportErrorMessage(node, MessageKind.RETURN_IN_GENERATOR,
             {'modifier': currentAsyncMarker});
       }
     }
     visit(node.expression);
     return const NoneResult();
   }
 
   ResolutionResult visitYield(Yield node) {
-    if (!currentAsyncMarker.isYielding) {
-      reporter.reportErrorMessage(node, MessageKind.INVALID_YIELD);
-    }
-    if (currentAsyncMarker.isAsync) {
-      coreClasses.streamClass.ensureResolved(resolution);
+    if (!compiler.backend.supportsAsyncAwait) {
+      reporter.reportErrorMessage(node.yieldToken,
+          MessageKind.ASYNC_AWAIT_NOT_SUPPORTED);
     } else {
-      coreClasses.iterableClass.ensureResolved(resolution);
+      if (!currentAsyncMarker.isYielding) {
+        reporter.reportErrorMessage(node, MessageKind.INVALID_YIELD);
+      }
+      if (currentAsyncMarker.isAsync) {
+        coreClasses.streamClass.ensureResolved(resolution);
+      } else {
+        coreClasses.iterableClass.ensureResolved(resolution);
+      }
     }
     visit(node.expression);
     return const NoneResult();
   }
 
   ResolutionResult visitRedirectingFactoryBody(RedirectingFactoryBody node) {
     if (!enclosingElement.isFactoryConstructor) {
       reporter.reportErrorMessage(
           node, MessageKind.FACTORY_REDIRECTION_IN_NON_FACTORY);
       reporter.reportHintMessage(
@@ skipping 109 matching lines @@
     return const NoneResult();
   }
 
   ResolutionResult visitThrow(Throw node) {
     registry.registerFeature(Feature.THROW_EXPRESSION);
     visit(node.expression);
     return const NoneResult();
   }
 
   ResolutionResult visitAwait(Await node) {
-    if (!currentAsyncMarker.isAsync) {
-      reporter.reportErrorMessage(node, MessageKind.INVALID_AWAIT);
+    if (!compiler.backend.supportsAsyncAwait) {
+      reporter.reportErrorMessage(node.awaitToken,
+          MessageKind.ASYNC_AWAIT_NOT_SUPPORTED);
+    } else {
+      if (!currentAsyncMarker.isAsync) {
+        reporter.reportErrorMessage(node, MessageKind.INVALID_AWAIT);
+      }
+      coreClasses.futureClass.ensureResolved(resolution);
     }
-    coreClasses.futureClass.ensureResolved(resolution);
     visit(node.expression);
     return const NoneResult();
   }
 
   ResolutionResult visitVariableDefinitions(VariableDefinitions node) {
     DartType type;
     if (node.type != null) {
       type = resolveTypeAnnotation(node.type);
     } else {
       type = const DynamicType();
@@ skipping 498 matching lines @@
     }
     registry.registerTargetOf(node, target);
     return const NoneResult();
   }
 
   registerImplicitInvocation(Selector selector) {
     registry.registerDynamicUse(new DynamicUse(selector, null));
   }
 
   ResolutionResult visitAsyncForIn(AsyncForIn node) {
-    if (!currentAsyncMarker.isAsync) {
-      reporter.reportErrorMessage(
-          node.awaitToken, MessageKind.INVALID_AWAIT_FOR_IN);
+    if (!compiler.backend.supportsAsyncAwait) {
+      reporter.reportErrorMessage(node.awaitToken,
+          MessageKind.ASYNC_AWAIT_NOT_SUPPORTED);
+    } else {
+      if (!currentAsyncMarker.isAsync) {
+        reporter.reportErrorMessage(
+            node.awaitToken, MessageKind.INVALID_AWAIT_FOR_IN);
+      }
+      registry.registerFeature(Feature.ASYNC_FOR_IN);
+      registry.registerDynamicUse(
+          new DynamicUse(Selectors.current, null));
+      registry.registerDynamicUse(
+          new DynamicUse(Selectors.moveNext, null));
     }
-    registry.registerFeature(Feature.ASYNC_FOR_IN);
-    registry.registerDynamicUse(new DynamicUse(Selectors.current, null));
-    registry.registerDynamicUse(new DynamicUse(Selectors.moveNext, null));
-
     visit(node.expression);
 
     Scope blockScope = new BlockScope(scope);
     visitForInDeclaredIdentifierIn(node.declaredIdentifier, node, blockScope);
     visitLoopBodyIn(node, node.body, blockScope);
     return const NoneResult();
   }
 
   ResolutionResult visitSyncForIn(SyncForIn node) {
     registry.registerFeature(Feature.SYNC_FOR_IN);
@@ skipping 456 matching lines @@
     }
     return const NoneResult();
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(
     DiagnosticReporter reporter, Node node, Scope scope, String name) {
   return Elements.unwrap(scope.lookup(name), reporter, node);
 }