Introduce an ElementTypeMask to recognize simple constraints like "the type of parameter foo is the…

sdk/lib/_internal/compiler/implementation/types/element_type_mask.dart

Index: sdk/lib/_internal/compiler/implementation/types/element_type_mask.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/types/element_type_mask.dart	(revision 0)
+++ sdk/lib/_internal/compiler/implementation/types/element_type_mask.dart	(revision 0)
@@ -0,0 +1,54 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of types;
+
+/**
+ * A [TypeMask] specific to an element: the return type for a
+ * function, or the type for a field.
+ */
+class ElementTypeMask extends ForwardingTypeMask {
+  final Element element;
+  // Callback function to fetch the actual inferred type of the
+  // element. It is used when a user wants to know about the type this
+  // [ForwardingTypeMask] forwards to.
+  final Function fetchForwardTo;
+  final bool isNullable;
+
+  ElementTypeMask(
+      this.fetchForwardTo, this.element, {this.isNullable: true});
+
+  bool get isElement => true;
+
+  TypeMask get forwardTo {
+    TypeMask forward = fetchForwardTo(element);
+    return isNullable ? forward.nullable() : forward.nonNullable();
+  }
+
+  bool operator==(other) {
+    if (other is! ElementTypeMask) return false;
+    return element == other.element && isNullable == other.isNullable;
+  }
+
+  bool equalsDisregardNull(other) {
+    if (other is! ElementTypeMask) return false;
+    return element == other.element;
+  }
+
+  TypeMask nullable() {
+    return isNullable
+        ? this
+        : new ElementTypeMask(fetchForwardTo, element, isNullable: true);
+  }
+
+  TypeMask nonNullable() {
+    return isNullable
+        ? new ElementTypeMask(fetchForwardTo, element, isNullable: false)
+        : this;
+  }
+
+  String toString() {
+    return 'Type for element $element';
+  }
+}