Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/scanner/partial_parser.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/partial_parser.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/partial_parser.dart b/sdk/lib/_internal/compiler/implementation/scanner/partial_parser.dart
index 98fc1ab3bf7344b2aece1bf2f08e7c0692f959b5..c6ccbf47c195864ec9bac83bfb4869b78336b903 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/partial_parser.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/partial_parser.dart
@@ -103,8 +103,29 @@ class PartialParser extends Parser {
     return endGroup;
   }
 
+  Token skipAsyncModifier(Token token) {
+    String value = token.stringValue;
+    if (identical(value, 'async')) {
+      token = token.next;
+      value = token.stringValue;
+
+      if (identical(value, '*')) {
+        token = token.next;
+      }
+    } else if (identical(value, 'sync')) {
+      token = token.next;
+      value = token.stringValue;
+
+      if (identical(value, '*')) {
+        token = token.next;
+      }
+    }
+    return token;
+  }
+
   Token parseFunctionBody(Token token, bool isExpression, bool allowAbstract) {
     assert(!isExpression);
+    token = skipAsyncModifier(token);
     String value = token.stringValue;
     if (identical(value, ';')) {
       if (!allowAbstract) {