Implement least upper bound.

sdk/lib/_internal/compiler/implementation/typechecker.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name => "Type checker";
 
@@ skipping 1404 matching lines @@
 
   DartType visitConditional(Conditional node) {
     Expression condition = node.condition;
     Expression thenExpression = node.thenExpression;
 
     checkCondition(condition);
 
     DartType thenType = analyzeInPromotedContext(condition, thenExpression);
 
     DartType elseType = analyzeNonVoid(node.elseExpression);
-    if (types.isSubtype(thenType, elseType)) {
-      return thenType;
-    } else if (types.isSubtype(elseType, thenType)) {
-      return elseType;
-    } else {
-      return objectType;
-    }
+    return compiler.types.computeLeastUpperBound(thenType, elseType);
   }
 
   visitStringInterpolation(StringInterpolation node) {
     node.visitChildren(this);
     return stringType;
   }
 
   visitStringInterpolationPart(StringInterpolationPart node) {
     node.visitChildren(this);
     return stringType;
@@ skipping 136 matching lines @@
   visitTypedef(Typedef node) {
     // Do not typecheck [Typedef] nodes.
   }
 
   visitNode(Node node) {
     compiler.internalError(
         'Unexpected node ${node.getObjectDescription()} in the type checker.',
         node: node);
   }
 }