Implement 'as' operator.

lib/compiler/implementation/scanner/parser.dart

Index: lib/compiler/implementation/scanner/parser.dart
diff --git a/lib/compiler/implementation/scanner/parser.dart b/lib/compiler/implementation/scanner/parser.dart
index 08b77c2665c205dfeb0409ac4be638fb6f75859b..dbe9c6ff7512186245112026fa292ef9334be101 100644
--- a/lib/compiler/implementation/scanner/parser.dart
+++ b/lib/compiler/implementation/scanner/parser.dart
@@ -934,6 +934,8 @@ class Parser {
           }
         } else if (info === IS_INFO) {
           token = parseIsOperatorRest(token);
+        } else if (info === AS_INFO) {
+          token = parseAsOperatorRest(token);
         } else if (info === QUESTION_INFO) {
           token = parseConditionalExpressionRest(token);
         } else {
@@ -1376,14 +1378,28 @@ class Parser {
     }
     token = parseType(token.next);
     listener.handleIsOperator(operator, not, token);
-    if (optional('is', token)) {
-      // The is-operator cannot be chained, but it can take part of
+    String value = token.stringValue;
+    if (value === 'is' || value === 'as') {
+      // The is- and as-operators cannot be chained, but they can take part of
       // expressions like: foo is Foo || foo is Bar.
       listener.unexpected(token);
     }
     return token;
   }
 
+  Token parseAsOperatorRest(Token token) {
+    assert(optional('as', token));
+    Token operator = token;
+    token = parseType(token.next);
+    listener.handleAsOperator(operator, token);
+    String value = token.stringValue;
+    if (value === 'is' || value === 'as') {
+      // The is- and as-operators cannot be chained.
+      listener.unexpected(token);
+    }
+    return token;
+  }
+
   Token parseVariablesDeclaration(Token token) {
     token = parseVariablesDeclarationNoSemicolon(token);
     return expectSemicolon(token);