When registering deferred constant, use canonical output unit.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 114bdcaaf8886442f87a8fc3b0fd7dbe63de1948..aa3bace09926215e4d1f4dadfbd8ecc01c5d9aab 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -226,7 +226,14 @@ class DeferredLoadTask extends CompilerTask {
       DeferredConstantValue constant, PrefixElement prefix) {
     OutputUnit outputUnit = new OutputUnit();
     outputUnit.imports.add(new _DeclaredDeferredImport(prefix.deferredImport));
-    _constantToOutputUnit[constant] = outputUnit;
+
+    // Check to see if there is already a canonical output unit registered.
+    OutputUnit representative = allOutputUnits.lookup(outputUnit);

[Siggi Cherem (dart-lang), 2016/09/07 18:10:44]

I see this logic replicated in a few places, maybe we should pull it out into a
helper.

[Siggi Cherem (dart-lang), 2016/09/07 18:14:09]

reading more into this, it feels like we are doing some kind of unification in
how we use the allOutputUnits and operator==. It might not matter too much in
terms of performance, but maybe it is worth replacing this with a union-find
datastructure?

[Harry Terkelsen, 2016/09/07 23:53:50]

Done.

[Harry Terkelsen, 2016/09/07 23:53:50]

added a todo

+    if (representative == null) {
+      representative = outputUnit;
+      allOutputUnits.add(representative);
+    }
+    _constantToOutputUnit[constant] = representative;
   }
 
   /// Answers whether [element] is explicitly deferred when referred to from
@@ -679,6 +686,11 @@ class DeferredLoadTask extends CompilerTask {
               Map<ConstantValue, OutputUnit> constantToOutputUnitBuilder =
                   new Map<ConstantValue, OutputUnit>();
 
+              // Add all constants that may have been registered during
+              // resolution with [registerConstantDeferredUse].

[Siggi Cherem (dart-lang), 2016/09/07 18:10:44]

would it make sense to store those somewhere else instead?

[Harry Terkelsen, 2016/09/07 23:53:50]

I wanted to do that at first, but it would mean having to differentiate between
when 'registerConstantDeferredUse' is called during resolution and when it is
called during codegen

+              constantToOutputUnitBuilder.addAll(_constantToOutputUnit);
+              _constantToOutputUnit.clear();
+
               // Reverse the mappings. For each element record an OutputUnit
               // collecting all deferred imports mapped to this element. Same
               // for constants.