Adds Dart_DefaultCanonicalizeUrl() to the dart embedding api.
Motivation:
As we try to get source reloading working for the standalone embedder, things get simpler if an isolate doesn't run Dart code while it is loading Dart code. We intend to solve this by moving the embedder tag handler calls to the service isolate. But making a blocking rpc into the service isolate whenever a url needs to be canonicalized during parsing seems like it would slow things down and make things complicated. By moving canonicalization into C++, we avoid this.
R=ahe@google.com, fschneider@google.com, johnmccutchan@google.com
Review URL: https://codereview.chromium.org/2011543002 .