[dart2js] expose minification as an environment variable

Change-Id: I52e8e01595fdeea029797cf569ef97978d8f1bbb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/346908
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

pkg/compiler/lib/src/options.dart

@@ -1226,6 +1226,7 @@ class CompilerOptions implements DiagnosticOptions {
 
     environment['dart.web.assertions_enabled'] = '$enableUserAssertions';
     environment['dart.tool.dart2js'] = '${true}';
+    environment['dart.tool.dart2js.minify'] = '$enableMinification';
     // Eventually pragmas and commandline flags should be aligned so that users
     // setting these flag is equivalent to setting the relevant pragmas
     // globally.