dart2js: Implement frequency based naming.

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index a36ee9ab9ba9beb8328e606bbd9292d64c67769c..52474246717f71c6f04db2b2d653d83023faf824 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -79,6 +79,7 @@ class Compiler extends leg.Compiler {
                 hasOption(options, '--disable-type-inference'),
             preserveComments: hasOption(options, '--preserve-comments'),
             useCpsIr: hasOption(options, '--use-cps-ir'),
+            useFrequencyNamer: hasOption(options, "--use-frequency-naming"),

[sra1, 2015/07/01 04:11:09]

Maybe move this next to --minify since that is what it modifies.
Pick a name relates it to minified code, e.g.  --minify-by-frequency

[herhut, 2015/07/02 08:54:51]

After talking to floitsch@, I have now enabled frequency based naming by default
and added --no-frequency-based-minification to enable the old behavior.

             verbose: hasOption(options, '--verbose'),
             sourceMapUri: extractUriOption(options, '--source-map='),
             outputUri: extractUriOption(options, '--out='),