Merge in the flutter gallery (#176)

gallery/codeviewer_cli/README.md

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+A command-line application to highlight dart source code.
+
+## Overview
+
+Code segments are highlighted before the app is compiled.
+This is done because the highlighting process can take 300ms to finish, creating a noticeable delay when the demo switches to code page.
+
+The highlighter takes all files in the `gallery/lib/demos/` folder and scans each.
+Highlighted code widgets are stored in the `gallery/lib/codeviewer/code_segments.dart` file.
+Under the root directory, run `make update-code-segments` to run the highlighter.
+
+Wrap a block of code with lines `// BEGIN yourDemoName` and `// END` to mark it for highlighting. The block in between, as well as any copyright notice and imports at the beginning of the file, are automatically taken and highlighted, and stored as `static TextSpan yourDemoName(BuildContext context)` in `gallery/lib/codeviewer/code_segments.dart`.
+To display the code, go to `gallery/lib/data/demos.dart`, and add `code: CodeSegments.yourDemoName,` to your `GalleryDemoConfiguration` object.
+
+## Multiple blocks of code
+
+Use the following method to join multiple blocks of code into a single segment:
+```
+// BEGIN yourDemo#2
+a();
+// END
+b();
+// BEGIN yourDemo#1
+c();
+// END
+```
+The generated code will be
+```
+c();
+a();
+```
+
+Code blocks can nest or overlap. In these cases, specify which file(s) to `END`.
+
+The following source file
+```
+// BEGIN demoOne
+a();
+// BEGIN demoTwo
+b();
+// END demoOne
+c();
+// END demoTwo
+```
+will create the following segments:
+(demoOne)
+```
+a();
+b();
+```
+(demoTwo)
+```
+b();
+c();
+```