Convert most Future-based code to async/await

Now that we only support Dart >=1.9, all supported Dart versions support
Isolate.kill().

Now that we use async/await, we're no longer compatible with Dart 1.8.

This moves the logic of starting the server and extracting its port
number to a utility function.

This makes it much easier to implement asynchronous methods and memoize
them or make them reentrant.

Convert JS stack traces into Dart stack traces.

Closes #21

This helps avoid issues where "localhost" means different things on
different platforms.

Closes #100

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1145483003

Don't wait for every suite to load at once—this basically ensures that
once there are enough, later ones will time out. Instead, only load
eight suites at once.

Closes #102

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1140033003

The label should be `transformers`, not `transformer`.

This releases 0.12.1.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1134193004

stack_trace 1.2.0 isn't compatible with test.

I tried `pub downgrade` and I got the error

 `Class 'LazyTrace' has no instance method 'foldFrames' with matching arguments.`

 After limiting stack_trace to '^1.2.1' it worked again.

Closes #73

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1124113002

Closes #104

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1123123002

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1112943002

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1116443002

This also releases 0.12.0-rc.1.

Closes #92

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1107743002

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1058973004

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1073983012

R=nweiz@google.com

Review URL: https://codereview.chromium.org//1103703002

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1107493002

R=nweiz@google.com

Review URL: https://codereview.chromium.org//1085363004

Closes #31

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1088933006

Windows now uses the expanded reporter by default, since it doesn't
support using \r to go to the beginning of the line.

Closes #85

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1100873005

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1099323002

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1077153006

Closes #87

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1101773002

Now, no test suite other than phantomjs_test.dart and
dartium_test.dart assume that phantomjs or dartium (respectively) are
installed. content_shell is still assumed to be installed, since it's
very useful for non-compilation-specific tests. More of those tests
have been moved to use content shell rather than chrome.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1088453004

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1096943003

The bots pass in relative paths, which breaks when we run processes
with new working directories.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1094533005

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1086083006

Closes #75

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1092153003

This output won't always be generated, and if we always declare it
"pub serve" will produce unsightly error messages when it would
conflict if generated.

See #84

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1097383002

This will never be as full-featured as running via "pub run", so we should frame
"pub run" as the canonical way to run tests.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1097113004

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1093213003

This allows platform-specific metadata to be declared.

See #75

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1097183003

This moves most of the functionality into reusable helper methods, to make it
easier to add new metadata in the future.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1093313002

Closes #82

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1085353004