Fixes https://github.com/flutter/flutter/issues/14751

* Fix snapshot fingerprint check for --preview-dart-2.

* Remove printTraces

These URLs all automatically redirect HTTP to HTTPS so makes sense to point users at the secure versions.

This will allow other tools to call into the roll_dev script
to know what version is about to be published without actually
publishing.

https://github.com/flutter/flutter/issues/14751

* fix potential call on null

* review

This is needed to support running with local engine's dart sdk.

This is follow-up to https://github.com/flutter/flutter/pull/14702.

* All lists produced by JSON parsing are List<dynamic>. If more
speficic type is required then they need to be explicitly cast, e.g.
using castFrom helper;
* Function of type (ByText) -> Finder is not a subtype of
(SerializableFinder) -> Finder because ByText is in the contravariant
position;
* In Dart 2 typed(any) should be used instead of any in mockito based
tests.

* Added sample code to AnimatedOpacity docs

Merging on red tree to get back to green.

- Remove redundant strong flag as it is the default for preview-dart-2
now

* Created plumbing but has stream problem

* testing with makePipe

* Trying pipe but not really getting anywhere

* works by repeatedly reading line

* Minor cleanup

* works

* Clean up pipe after use.

* Move the last status forward

* Make sure failed script commands bubble up

* Small glow overscroll dartdoc addition

* Directly specify [MaterialApp]

* review

This reverts commit b3e4976c.

Reports from that commit included:

* Doctor hanging
* Doctor prompting to install JDK
* AnsiStatus.cancel() printing elapsed time and extra newline
* Printing extra spinner character at the end of each line

up the latest Dart roll

- Resubmit https://github.com/flutter/flutter/pull/14392 to fix
compilation errors

If compiler failed to produce Kernel binary then compile(...) returns
null to the caller. If we don't check for null we end up trying to
run file called "null" which causes a very confusing crash of the
flutter_tester.

* Use already downloaded dart sdk for frontend_server.

This is follow-up to https://github.com/flutter/flutter/pull/14610

* Remove reference to engine dart-sdk from artifacts

* Remove unused imports

* WIP to switch back to solution where dart sdk is downloaded into bin/cache/dart-sdk, but it is 'flutter precache -a' that can download all dart sdks for other platforms under bin/cache/artifacts/engine/<platform>/dart-sdk

* Download all dart sdks when 'flutter precache -a'