Wire up more of the DebuggerAgent

The debugger can now correctly break on exceptions
and show the corresponding line in the inspector.

It correctly understands which scripts are internal
to sky and does not pause during them.

There is still a ton to make work here
(including stacktraces which I have not tested),
but basic functionality seems to work.

The current implementation is not smart enough to
unpause the inspector when the frontend disconnects.

BUG=434510,434513
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/727593004

engine/src/flutter/tests/inspector/page-agent-get-resource-expected.txt

@@ -0,0 +1 @@
+Got message!