Check for canBePrimitiveNumber when doing the intersection on a number type.

tests/language/type_intersection_test.dart

+// 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.
+
+// Regression test for dart2js that used to consider that the
+// intersection of [Comparable] and [num] is conflicting.
+
+class A {
+  foo(a, Comparable b) => a == b;
+  bar(a, Comparable b) => b == a;
+}
+
+main() {
+  Expect.isFalse(new A().foo(1, 'foo'));
+  Expect.isTrue(new A().foo(1, 1));
+  Expect.isFalse(new A().bar(1, 'foo'));
+  Expect.isTrue(new A().bar(1, 1));
+}