Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/lib/js_helper.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index ee23e0dbb08ec629f16073732225897c4dc55c7f..f083720bbac75f2070f36fed8fecc569b3258af9 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -1541,18 +1541,44 @@ getArguments(var type) => JS('var', r'#.slice(1)', type);
 getField(var object, var name) => JS('var', r'#[#]', object, name);
 
 /**
+ * Tests whether the Dart object [o] is a subtype of the runtime type
+ * representation [t], which is a type representation as described in the
+ * comment on [isSubtype].
+ */
+bool objectIsSubtype(Object o, var t) {
+  if (JS('bool', '# == null', o) || JS('bool', '# == null', t)) return true;
+
+  var nativeCheck = getField(t, '\$nativeCheck');
+  if (nativeCheck != null) {
+    return invoke(nativeCheck, [o]);
+  } else {
+    var type = o;

[ngeoffray, 2013/02/21 10:26:18]

Please add a comment on why o could be the type.

[karlklose, 2013/02/21 14:48:44]

Done.

+    var rti = getRuntimeTypeInfo(o);
+    if (JS('bool', '# != null', rti)) {

[ngeoffray, 2013/02/21 10:26:18]

Add a comment on why it could be null.

[karlklose, 2013/02/21 14:48:44]

Done.

+      // Make a copy of the type arguments and insert [o] in the first position
+      // to create a compound type representation.
+      type = JS('List', '#.slice().splice(0, 0, #)', rti, o);
+    }
+    return isSubtype(type, t);
+  }
+}
+
+
+/**
  * Check whether the type represented by [s] is a subtype of the type
  * represented by [t].
  *
  * Type representations can be:
  *  1) a JavaScript constructor for a class C: the represented type is the raw
  *     type C.
- *  2) a JavaScript object: this represents a class for which there is no
+ *  2) a Dart object.

[ngeoffray, 2013/02/21 10:26:18]

That is very confusing, how can a Dart object be a type?

[karlklose, 2013/02/21 14:48:44]

As discussed, the checks and substitutions are on its prototype so it serves as
a representation of its class.

+ *  3) a JavaScript object: this represents a class for which there is no
  *     JavaScript constructor, because it is only used in type arguments or it
  *     is native. The represented type is the raw type of this class.
- *  3) a JavaScript array: the first entry is of type 1 or 2 and identifies the
- *     class of the type and the rest of the array are the type arguments.
- *  4) [:null:]: the dynamic type.
+ *  4) a JavaScript array: the first entry is of type 1, 2 or 3 and contains the
+ *     subtyping flags and the substitution of the type and the rest of the
+ *     array are the type arguments.
+ *  5) [:null:]: the dynamic type.
  */
 bool isSubtype(var s, var t) {
   // If either type is dynamic, [s] is a subtype of [t].