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 844ee78c9aeec4f7d3beff36806de95ad2b3fdcf..a1d94d2aef76e0d53d1aa9d01da7d2018b28d2e9 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -1522,6 +1522,31 @@ 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].
+ */
+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 {
+    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].
  *