dart2js cps: Generate fast checks for 'is String' and 'is bool'.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 831db16e27bf98e3fe4845846d609fb69204f418..878145dafe31bbaa0338675b72218dcdf8a3a529 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -358,12 +358,28 @@ class CodeGenerator extends tree_ir.StatementVisitor
 
       // Handle some special checks against classes that exist only in
       // the compile-time class hierarchy, not at runtime.
+      // TODO(sra): Is this correct? The field tests are only valid with the
+      // precondition that [value] is an Array. They will crash on `null`.
       if (clazz == glue.jsExtendableArrayClass) {
+        assert(node.isTypeTest);
         return js.js(r'!#.fixed$length', <js.Expression>[value]);
       } else if (clazz == glue.jsMutableArrayClass) {
+        assert(node.isTypeTest);
         return js.js(r'!#.immutable$list', <js.Expression>[value]);
       }
 
+      if (glue.isStringClass(clazz)) {
+        if (node.isTypeTest) {
+          return js.js(r'typeof # === "string"', <js.Expression>[value]);
+        }
+        // TODO(sra): Implement fast cast via calling 'stringTypeCast'.
+      } else if (glue.isBoolClass(clazz)) {
+        if (node.isTypeTest) {
+          return js.js(r'typeof # === "boolean"', <js.Expression>[value]);
+        }
+        // TODO(sra): Implement fast cast via calling 'boolTypeCast'.
+      }
+
       // The helper we use needs the JSArray class to exist, but for some
       // reason the helper does not cause this dependency to be registered.
       // TODO(asgerf): Most programs need List anyway, but we should fix this.