Allow embedders to provide custom message delivery for an isolate.

runtime/vm/assert.h

Index: runtime/vm/assert.h
===================================================================
--- runtime/vm/assert.h	(revision 513)
+++ runtime/vm/assert.h	(working copy)
@@ -39,6 +39,9 @@
   template<typename E, typename A>
   void Equals(const E& expected, const A& actual);
 
+  template<typename E, typename A>
+  void NotEquals(const E& not_expected, const A& actual);
+
   template<typename E, typename A, typename T>
   void FloatEquals(const E& expected, const A& actual, const T& tol);
 
@@ -95,6 +98,17 @@
 }
 
 
+template<typename E, typename A>
+void DynamicAssertionHelper::NotEquals(const E& not_expected,
+                                       const A& actual) {
+  if (actual != not_expected) return;
+  std::stringstream ness;
+  ness << not_expected;
+  std::string nes = ness.str();
+  Fail("did not expect: <%s>", nes.c_str());
+}
+
+
 template<typename E, typename A, typename T>
 void DynamicAssertionHelper::FloatEquals(const E& expected,
                                          const A& actual,
@@ -225,6 +239,9 @@
 #define EXPECT_EQ(expected, actual)                                            \
   dart::Expect(__FILE__, __LINE__).Equals((expected), (actual))
 
+#define EXPECT_NE(not_expected, actual)                                        \
+  dart::Expect(__FILE__, __LINE__).NotEquals((not_expected), (actual))
+
 #define EXPECT_FLOAT_EQ(expected, actual, tol)                                 \
   dart::Expect(__FILE__, __LINE__).FloatEquals((expected), (actual), (tol))