Moves: test/sdk --> tool/input_sdk_src
Also moves: test/sdk/lib/_internal/js_lib --> tool/input_sdk_patch
we'll eventually need to customize that code
Adds tool/patch_sdk.dart, an offline transformation step that produces: test/generated_sdk
Essentially this merges the "external" declarations and the @patch syntactically. There's a comment in tool/patch_sdk.dart explaining the rationale behind this approach.
There's still *lots* to do in the generated code. This does none of that. The only new code is tool/patch_sdk.dart and the minimum changes to test/codegen_test.dart and js_codegen.dart to get test.sh running and debuggable.
Tracking bug for SDK is https://github.com/dart-lang/dart-dev-compiler/issues/58R=sigmund@google.com, vsm@google.com
Review URL: https://chromereviews.googleplex.com/157137013