Support async/await in the dart2js frontend.

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

Index: sdk/lib/_internal/compiler/implementation/scanner/token.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/token.dart b/sdk/lib/_internal/compiler/implementation/scanner/token.dart
index 0b474d417bde822326dffd1e7b4e4d55ddec8f5f..483eda9fcf3d68aed4932ae4bf14a5530b0870ef 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/token.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/token.dart
@@ -169,6 +169,15 @@ abstract class Token implements Spannable {
   int get hashCode => computeHashCode(charOffset, info, value);
 }
 
+/// A pair of tokens marking the beginning and the end of a span. Use for error
+/// reporting.
+class TokenPair implements Spannable {
+  final Token begin;
+  final Token end;
+
+  TokenPair(this.begin, this.end);
+}
+
 /**
  * A [SymbolToken] represents the symbol in its precendence info.
  * Also used for end of file with EOF_INFO.