Add ability for Function to generate source

runtime/vm/compiler_test.cc

Index: runtime/vm/compiler_test.cc
diff --git a/runtime/vm/compiler_test.cc b/runtime/vm/compiler_test.cc
index fcee41e6f55a58bb31abe0028b9bc60fcbdaa64d..08e2285d08e39e77bbf20cd04a7aedd67acea251 100644
--- a/runtime/vm/compiler_test.cc
+++ b/runtime/vm/compiler_test.cc
@@ -50,7 +50,8 @@ TEST_CASE(CompileFunction) {
   Function& function_foo =
       Function::Handle(cls.LookupStaticFunction(function_foo_name));
   EXPECT(!function_foo.IsNull());
-
+  String& function_source = String::Handle(function_foo.GetSource());
+  EXPECT_STREQ("static foo() { return 42; }\n  ", function_source.ToCString());
   EXPECT(CompilerTest::TestCompileFunction(function_foo));
   EXPECT(function_foo.HasCode());
 
@@ -61,6 +62,9 @@ TEST_CASE(CompileFunction) {
 
   EXPECT(CompilerTest::TestCompileFunction(function_moo));
   EXPECT(function_moo.HasCode());
+  function_source = function_moo.GetSource();
+  EXPECT_STREQ("static moo() {\n    // A.foo();\n  }\n",
+               function_source.ToCString());
 }