Implement 'as' operator.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
diff --git a/lib/compiler/implementation/lib/js_helper.dart b/lib/compiler/implementation/lib/js_helper.dart
index 55f0bbabd8a21b56b65c26a32c6b70e68f74b8cd..298d58a45c2971ba0166f35db3bca28f7febe746 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -951,10 +951,15 @@ getRuntimeTypeInfo(target) {
   return JS('var', @'#.builtin$typeInfo', target);
 }
 
+
+class CastException extends TypeError {
+  CastException(String message): super(message);
+}
+
 /**
  * The following methods are called by the runtime to implement
- * checked mode. We specialize each primitive type (eg int, bool), and
- * use the compiler's convention to do is checks on regular objects.
+ * checked mode and casts. We specialize each primitive type (eg int, bool), and
+ * use the compiler's convention to do is-checks on regular objects.
  */
 stringTypeCheck(value) {
   if (value === null) return value;
@@ -962,42 +967,84 @@ stringTypeCheck(value) {
   throw new TypeError('$value does not implement String');
 }
 
+stringTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is String) return value;
+  throw new CastException('$value does not implement String');
+}
+
 doubleTypeCheck(value) {
   if (value === null) return value;
   if (value is double) return value;
   throw new TypeError('$value does not implement double');
 }
 
+doubleTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is double) return value;
+  throw new CastException('$value does not implement double');
+}
+
 numTypeCheck(value) {
   if (value === null) return value;
   if (value is num) return value;
   throw new TypeError('$value does not implement num');
 }
 
+numTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is num) return value;
+  throw new CastException('$value does not implement num');
+}
+
 boolTypeCheck(value) {
   if (value === null) return value;
   if (value is bool) return value;
   throw new TypeError('$value does not implement bool');
 }
 
+boolTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is bool) return value;
+  throw new CastException('$value does not implement bool');
+}
+
 functionTypeCheck(value) {
   if (value === null) return value;
   if (value is Function) return value;
   throw new TypeError('$value does not implement Function');
 }
 
+functionTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is Function) return value;
+  throw new CastException('$value does not implement Function');
+}
+
 intTypeCheck(value) {
   if (value === null) return value;
   if (value is int) return value;
   throw new TypeError('$value does not implement int');
 }
 
+intTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is int) return value;
+  throw new CastException('$value does not implement int');
+}
+
 void propertyTypeError(value, property) {
   // Cuts the property name to the class name.
   String name = property.substring(3, property.length);
   throw new TypeError('$value does not implement $name');
 }
 
+void propertyTypeCastError(value, property) {
+  // Cuts the property name to the class name.
+  String name = property.substring(3, property.length);
+  throw new CastException('$value does not implement $name');
+}
+
 /**
  * For types that are not supertypes of native (eg DOM) types,
  * we emit a simple property check to check that an object implements
@@ -1009,6 +1056,12 @@ propertyTypeCheck(value, property) {
   propertyTypeError(value, property);
 }
 
+propertyTypeCast(Object value, Object property) {
+  if (value === null) throw new NullPointerException();
+  if (JS('bool', '!!#[#]', value, property)) return value;
+  propertyTypeCastError(value, property);
+}
+
 /**
  * For types that are supertypes of native (eg DOM) types, we emit a
  * call because we cannot add a JS property to their prototype at load
@@ -1023,6 +1076,15 @@ callTypeCheck(value, property) {
   propertyTypeError(value, property);
 }
 
+callTypeCast(value, property) {
+  if (value === null) throw new NullPointerException();
+  if ((JS('String', 'typeof #', value) === 'object')
+      && JS('bool', '#[#]()', value, property)) {
+    return value;
+  }
+  propertyTypeCastError(value, property);
+}
+
 /**
  * Specialization of the type check for String and its supertype
  * since [value] can be a JS primitive.
@@ -1034,6 +1096,13 @@ stringSuperTypeCheck(value, property) {
   propertyTypeError(value, property);
 }
 
+stringSuperTypeCast(value, property) {
+  if (value === null) throw new NullPointerException();
+  if (value is String) return value;
+  if (JS('bool', '!!#[#]', value, property)) return value;
+  propertyTypeCastError(value, property);
+}
+
 stringSuperNativeTypeCheck(value, property) {
   if (value === null) return value;
   if (value is String) return value;
@@ -1041,6 +1110,13 @@ stringSuperNativeTypeCheck(value, property) {
   propertyTypeError(value, property);
 }
 
+stringSuperNativeTypeCast(value, property) {
+  if (value === null) throw new NullPointerException();
+  if (value is String) return value;
+  if (JS('bool', '#[#]()', value, property)) return value;
+  propertyTypeCastError(value, property);
+}
+
 /**
  * Specialization of the type check for List and its supertypes,
  * since [value] can be a JS array.
@@ -1051,6 +1127,12 @@ listTypeCheck(value) {
   throw new TypeError('$value does not implement List');
 }
 
+listTypeCast(value) {
+  if (value === null) throw new NullPointerException();
+  if (value is List) return value;
+  throw new CastException('$value does not implement List');
+}
+
 listSuperTypeCheck(value, property) {
   if (value === null) return value;
   if (value is List) return value;
@@ -1058,9 +1140,23 @@ listSuperTypeCheck(value, property) {
   propertyTypeError(value, property);
 }
 
+listSuperTypeCast(value, property) {
+  if (value === null) throw new NullPointerException();
+  if (value is List) return value;
+  if (JS('bool', '!!#[#]', value, property)) return value;
+  propertyTypeCastError(value, property);
+}
+
 listSuperNativeTypeCheck(value, property) {
   if (value === null) return value;
   if (value is List) return value;
   if (JS('bool', '#[#]()', value, property)) return value;
   propertyTypeError(value, property);
 }
+
+listSuperNativeTypeCast(value, property) {
+  if (value === null) throw new NullPointerException();
+  if (value is List) return value;
+  if (JS('bool', '#[#]()', value, property)) return value;
+  propertyTypeCastError(value, property);
+}