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 61f532e788af9c9594e51e7f7ac3cdd8436a34d8..2ea176fd511ecd90f7d6b57f24021716d116362a 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -1521,6 +1521,31 @@ 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].

[ngeoffray, 2013/02/18 09:27:58]

Is [t] the same thing as [t] in isSubtype?

[karlklose, 2013/02/18 16:02:01]

Yes, added a comment.

+ */
+bool objectIsSubtype(Object o, var t) {

[ngeoffray, 2013/02/18 09:27:58]

Why isn't o.runtimeType sufficient?

[karlklose, 2013/02/18 16:02:01]

No, runtimeType will return a Type that is only useful for comparison. It does
not support subtype checks.
(Additionally, it turns the runtimeType support on, which is not free ATM.)

+  if (JS('bool', '# == null', o) || JS('bool', '# == null', t)) return true;
+
+  var nativeCheck = getField(t, '\$nativeCheck');
+  if (nativeCheck != null) {
+    return invoke(nativeCheck, [o]);
+  } else {

[ngeoffray, 2013/02/18 09:27:58]

Why are you not calling isSubtype here? Can't you get the runtime type
representation of o?

[karlklose, 2013/02/18 16:02:01]

I could, but not directly, I would have to construct it; I think that would be
more complicated.

[ngeoffray, 2013/02/19 09:00:41]

Why would you have to construct it? Isn't it set when the object is being
allocated? If not, don't you already have helpers to create it?

[karlklose, 2013/02/19 12:39:28]

Done.

+    bool hasArguments = isJsArray(t);
+    String typeOfT = runtimeTypeToString(hasArguments ? t[0] : t);
+    // Check for a subtyping flag.
+    String test = '${JS_OPERATOR_IS_PREFIX()}$typeOfT';
+    if (getField(o, test) == null) return false;
+    if (!hasArguments) return true;
+    // Get the necessary substitution of the type arguments, if there is one.
+    String field = '${JS_OPERATOR_AS_PREFIX()}$typeOfT';
+    var substitution = getField(o, field);
+    // Recursively check the type arguments.
+    return checkArguments(substitution, getRuntimeTypeInfo(o), getArguments(t));
+  }
+}
+
+/**
  * Check whether the type represented by [s] is a subtype of the type
  * represented by [t].
  *