Clarify in the docs about what an invalid selection means, contrasted with a 0,0 selection.

Before this change, it was possible to select and copy obscured text from a text field.

This PR changes things so that:

- Obscured text fields don't allow copy or cut.
- If a field is both obscured and read-only, then selection is disabled as well (if you can't modify it, and can't copy it, there's no point in selecting it).

Updates the platform shims in dev/manual_tests so that Windows and Linux can be built. I had to update the Windows shims, because I was unable to build a Windows app there.

Also updates the analyze.dart script to report all license issues simultaneously instead of just dying after the first failure.

The only substantive code changes are in dev/bots/analyze.dart and dev/bots/test/analyze_test.dart

New parameter for InteractiveViewer: scaleFactor, which allows control over the speed of zooming.

* 1f60bfcbb Fix missing shcore.dll error on Windows 7 (flutter/engine#30699)

* 4c303676d Roll Fuchsia Mac SDK from G04Sc3__F... to WjrtT4DAB... (flutter/engine#30700)

* 2bf6b01be Migrate keyboard_key_handler_unittests to keyboard_win32_unittests (flutter/engine#30615)

* ed6868f7b Update //third_party/glfw from Aug 2016 to Jan 2019 (revision 78e6a006) (flutter/engine#30609)

* 7fae72abc Roll Skia from 88c5af7ecd72 to 8e6b60f27da3 (3 revisions) (flutter/engine#30703)

* b01a5a938 Roll Fuchsia Mac SDK from WjrtT4DAB... to fSRFvvC1c... (flutter/engine#30705)

* 63f9190a4 Roll Skia from 8e6b60f27da3 to f5853ff6525b (1 revision) (flutter/engine#30706)

* 7467eac56 Roll Skia from f5853ff6525b to db2b8eb98f14 (1 revision) (flutter/engine#30708)

* 936c3a0a2 Roll Skia from db2b8eb98f14 to 426f7b5ad606 (2 revisions) (flutter/engine#30710)

* 9524ec80f Roll Skia from 426f7b5ad606 to a5d4541ae556 (3 revisions) (flutter/engine#30712)

* e8bb6866b Roll Skia from a5d4541ae556 to 96e4053be7a5 (3 revisions) (flutter/engine#30714)

* d301340f5 Roll Skia from 96e4053be7a5 to ce49ff6520fa (4 revisions) (flutter/engine#30715)

* 38a007d12 Roll Dart SDK from f59531cc2973 to 5f2c54513b7d (13 revisions) (flutter/engine#30716)

* 305dcd5a2 Roll Skia from ce49ff6520fa to 73657689b6cb (5 revisions) (flutter/engine#30718)

* d930dce5b [web] allow customizing Ahem URL using FontManifest.json (flutter/engine#30719)

* a0bbfeb83 Roll Skia from 73657689b6cb to 72412a86725a (3 revisions) (flutter/engine#30721)

* d53b3c247 Fix aot builds in the dart_runner for fuchsia (flutter/engine#30722)

* 56816a0ba Roll Skia from 72412a86725a to 2f3891bf76f3 (1 revision) (flutter/engine#30723)

* 496e05526 Roll Dart SDK from 5f2c54513b7d to 8a8fb0972646 (1 revision) (flutter/engine#30725)

* 685d2208e Bump Robolectric to 4.7.3 (flutter/engine#30437)

* f8a8276f9 Roll Fuchsia Mac SDK from fSRFvvC1c... to cSwk_StnQ... (flutter/engine#30727)

* 594238c30 Revert Dart SDK to f59531cc2973 (flutter/engine#30729)

Commit on red to fix the tree.