dart2js: Support const symbols in the startup emitter.

sdk/lib/_internal/js_runtime/lib/shared/embedded_names.dart

Index: sdk/lib/_internal/js_runtime/lib/shared/embedded_names.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/shared/embedded_names.dart b/sdk/lib/_internal/js_runtime/lib/shared/embedded_names.dart
index a71d7067f924b5539c6d573e1348acd20a1d5bd9..b08e3a3d257d03de9151b2e97772536d18f9b1fa 100644
--- a/sdk/lib/_internal/js_runtime/lib/shared/embedded_names.dart
+++ b/sdk/lib/_internal/js_runtime/lib/shared/embedded_names.dart
@@ -79,8 +79,10 @@ const MANGLED_GLOBAL_NAMES = 'mangledGlobalNames';
 
 /// A JS map from mangled instance names to their unmangled names.
 ///
-/// If the program does not use reflection, this embedded global may be empty
-/// (but not null or undefined).
+/// This embedded global is mainly used for reflection, but is also used to
+/// map const-symbols (`const Symbol('x')`) to the mangled instance names.
+///
+/// This embedded global may be empty (but not null or undefined).
 const MANGLED_NAMES = 'mangledNames';
 
 /// A JS map from dispatch tags (usually constructor names of DOM classes) to