Support customizing API (and hence mock) compiler to minify the output.

lib/compiler/implementation/apiimpl.dart

Index: lib/compiler/implementation/apiimpl.dart
diff --git a/lib/compiler/implementation/apiimpl.dart b/lib/compiler/implementation/apiimpl.dart
index b162a506875d69d023936f8bc2b786a8e9507163..376a136b54a437dc62a203e841550d0b91305dec 100644
--- a/lib/compiler/implementation/apiimpl.dart
+++ b/lib/compiler/implementation/apiimpl.dart
@@ -29,7 +29,8 @@ class Compiler extends leg.Compiler {
           tracer: new ssa.HTracer(),
           enableTypeAssertions: options.indexOf('--enable-checked-mode') != -1,
           enableUserAssertions: options.indexOf('--enable-checked-mode') != -1,
-          emitJavascript: options.indexOf('--output-type=dart') == -1);
+          emitJavascript: options.indexOf('--output-type=dart') == -1,
+          minify: options.indexOf('--minify') != -1);
 
   elements.LibraryElement scanBuiltinLibrary(String path) {
     Uri uri = libraryRoot.resolve(DART2JS_LIBRARY_MAP[path].libraryPath);