Almost entirely `dartfmt --fix -w .`

* Update int64.dart

* Update int64_test.dart

* Update int64_test.dart

* Update pubspec.yaml

* Update int64.dart

* Update int64.dart

* Update int64_test.dart

* Update int64.dart

* Update CHANGELOG.md

* Update int64_test.dart

* Update int64.dart

Fixes https://github.com/dart-lang/fixnum/issues/41

See dart-lang/sdk#32030

Approved by Google OSS

Without this fix, new Int64(3) << 64 == Int64(3). This ensures that the result is 0 in such cases.

* Int64(int value) should not be sensitive to rounding errors

* fix error in test

* fix constructor

* new package version number

Cleanup

BUG=
R=rnystrom@google.com

Review-Url: https://codereview.chromium.org//2979043002 .

Also adding a CHANGELOG and analysis_options since it's strong mode
clean now.

R=cbracken@google.com, leafp@google.com

Review URL: https://codereview.chromium.org//1963283002 .

Includes performance fixes and error tests.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1834783004 .