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..b4e586a8de10b3b67f5837caee73943ee882e17b 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -222,11 +222,23 @@ class DeferredLoadTask extends CompilerTask {
     return outputUnitTo.imports.containsAll(outputUnitFrom.imports);
   }
 
+  // TODO(het): use a union-find to canonicalize output units
+  OutputUnit _getCanonicalUnit(OutputUnit outputUnit) {
+    OutputUnit representative = allOutputUnits.lookup(outputUnit);
+    if (representative == null) {
+      representative = outputUnit;
+      allOutputUnits.add(representative);
+    }
+    return representative;
+  }
+
   void registerConstantDeferredUse(
       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.
+    _constantToOutputUnit[constant] = _getCanonicalUnit(outputUnit);
   }
 
   /// Answers whether [element] is explicitly deferred when referred to from
@@ -679,6 +691,11 @@ class DeferredLoadTask extends CompilerTask {
               Map<ConstantValue, OutputUnit> constantToOutputUnitBuilder =
                   new Map<ConstantValue, OutputUnit>();
 
+              // Add all constants that may have been registered during
+              // resolution with [registerConstantDeferredUse].
+              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.
@@ -721,21 +738,11 @@ class DeferredLoadTask extends CompilerTask {
               // to, and canonicalize them.
               elementToOutputUnitBuilder
                   .forEach((Element element, OutputUnit outputUnit) {
-                OutputUnit representative = allOutputUnits.lookup(outputUnit);
-                if (representative == null) {
-                  representative = outputUnit;
-                  allOutputUnits.add(representative);
-                }
-                _elementToOutputUnit[element] = representative;
+                _elementToOutputUnit[element] = _getCanonicalUnit(outputUnit);
               });
               constantToOutputUnitBuilder
                   .forEach((ConstantValue constant, OutputUnit outputUnit) {
-                OutputUnit representative = allOutputUnits.lookup(outputUnit);
-                if (representative == null) {
-                  representative = outputUnit;
-                  allOutputUnits.add(representative);
-                }
-                _constantToOutputUnit[constant] = representative;
+                _constantToOutputUnit[constant] = _getCanonicalUnit(outputUnit);
               });
 
               // Generate a unique name for each OutputUnit.