Enable colors on-demand.

pkg/front_end/lib/src/fasta/compiler_context.dart

Index: pkg/front_end/lib/src/fasta/compiler_context.dart
diff --git a/pkg/front_end/lib/src/fasta/compiler_context.dart b/pkg/front_end/lib/src/fasta/compiler_context.dart
index 556ad819059e6da09bdd64219247a3be0e88fe22..040858c81ca4181fce81dd958968a743c8e42dfe 100644
--- a/pkg/front_end/lib/src/fasta/compiler_context.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_context.dart
@@ -10,6 +10,8 @@ import 'package:kernel/ast.dart' show Source;
 
 import 'compiler_command_line.dart' show CompilerCommandLine;
 
+import 'colors.dart' show computeEnableColors;
+
 final Object compilerContextKey = new Object();
 
 final CompilerContext rootContext =
@@ -20,6 +22,8 @@ class CompilerContext {
 
   final Map<String, Source> uriToSource = <String, Source>{};
 
+  bool enableColorsCached = null;
+
   CompilerContext(this.options);
 
   static CompilerContext get current {
@@ -33,4 +37,8 @@ class CompilerContext {
     CompilerContext c = new CompilerContext(cl);
     return runZoned(() => action(c), zoneValues: {compilerContextKey: c});
   }
+
+  static bool get enableColors {
+    return current.enableColorsCached ??= computeEnableColors(current);
+  }
 }