Issue 6399. Don't fall back to 'dynamic' when variable has explicit type.

compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java

Index: compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
diff --git a/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java b/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
index ba5e65a64ef635cc6d4d698b14db88f43f391825..8aee54083670c0dd12cd5bea457322354c026cba 100644
--- a/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
+++ b/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
@@ -774,7 +774,9 @@ public class TypeAnalyzer implements DartCompilationPhase {
             // if we fell back to Dynamic, keep it
           } else {
             Type unionType = getUnionType(currentType, inferredType);
-            unionType = Types.makeInferred(unionType);
+            if (unionType != currentType) {
+              unionType = Types.makeInferred(unionType);
+            }
             blockOldTypes.getFirst().rememberOldType(element, element.getType());
             Elements.setType(element, unionType);
           }
@@ -785,21 +787,25 @@ public class TypeAnalyzer implements DartCompilationPhase {
        * @return the {@link Type} which is both "a" and "b" types. May be "dynamic" if "a" and "b"
        *         don't form hierarchy.
        */
-      Type getUnionType(Type a, Type b) {
-        if (TypeKind.of(a) == TypeKind.DYNAMIC) {
-          return b;
+      Type getUnionType(Type curType, Type newType) {
+        if (TypeKind.of(curType) == TypeKind.DYNAMIC) {
+          return newType;
         }
-        if (TypeKind.of(b) == TypeKind.DYNAMIC) {
-          return a;
+        if (TypeKind.of(newType) == TypeKind.DYNAMIC) {
+          return curType;
         }
-        if (types.isSubtype(a, b)) {
-          return a;
+        if (types.isSubtype(curType, newType)) {
+          return curType;
         }
-        if (types.isSubtype(b, a)) {
-          return b;
+        if (types.isSubtype(newType, curType)) {
+          return newType;
         }
-        // TODO(scheglov) return union of types, but this is not easy
-        return dynamicType;
+        // if InterfaceType, use union
+        if (curType instanceof InterfaceType && newType instanceof InterfaceType) {
+          return types.unionTypes(ImmutableList.of((InterfaceType) curType, (InterfaceType) newType));
+        }
+        // keep type as is
+        return curType;
       }
 
       void restore() {