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..6485a8203d4af428b52bc00d5d1a25fd1ccd17dc 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -222,11 +222,24 @@ 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.
+    OutputUnit representative = _getCanonicalUnit(outputUnit);
+    _constantToOutputUnit[constant] = representative;
   }
 
   /// Answers whether [element] is explicitly deferred when referred to from
@@ -679,6 +692,11 @@ 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.
@@ -721,20 +739,12 @@ 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);
-                }
+                OutputUnit representative = _getCanonicalUnit(outputUnit);

[Siggi Cherem (dart-lang), 2016/09/07 23:57:50]

now that the logic is pulled out, consider removing the intermediate variable :)
(same elsewhere)

[Harry Terkelsen, 2016/09/08 00:01:29]

Done.

                 _elementToOutputUnit[element] = representative;
               });
               constantToOutputUnitBuilder
                   .forEach((ConstantValue constant, OutputUnit outputUnit) {
-                OutputUnit representative = allOutputUnits.lookup(outputUnit);
-                if (representative == null) {
-                  representative = outputUnit;
-                  allOutputUnits.add(representative);
-                }
+                OutputUnit representative = _getCanonicalUnit(outputUnit);
                 _constantToOutputUnit[constant] = representative;
               });