Implement export clause in VM

tests/language/import_combinators_test.dart

Index: tests/language/import_combinators_test.dart
===================================================================
--- tests/language/import_combinators_test.dart	(revision 13255)
+++ tests/language/import_combinators_test.dart	(working copy)
@@ -5,11 +5,20 @@
 // Dart test program importing with show/hide combinators.
 
 import "import1_lib.dart" show hide, show hide ugly;
+import "export1_lib.dart";
+import "dart:math" as M show E;
 
 part "import_combinators_part.dart";
 
 main() {
-  print(hide);
-  print(show);
-  print(lookBehindCurtain());
+  Expect.equals("hide", hide);
+  Expect.equals("show", show);
+  // Top-level function from part, refers to imported variable show.
+  Expect.equals("show", lookBehindCurtain());
+  // Top-level variable E from export1_lib.dart.
+  Expect.equals("E", E);
+  // Top-level variable E imported from dart:math.
+  Expect.equals(2.718281828459045, M.E);
+  // Constant LN2 from math library, re-exported by export1_lib.dart.
+  Expect.equals(0.6931471805599453, LN2);
 }