Fix various memory leaks in unit tests detected by a new version of ASAN.

runtime/vm/snapshot_test.cc

Index: runtime/vm/snapshot_test.cc
diff --git a/runtime/vm/snapshot_test.cc b/runtime/vm/snapshot_test.cc
index 79b5b1b907a8b9599fcaff1b479a69652c923969..fec51a0db35b7ed39d5473956c80e4726fefe83d 100644
--- a/runtime/vm/snapshot_test.cc
+++ b/runtime/vm/snapshot_test.cc
@@ -139,6 +139,8 @@ static void CheckEncodeDecodeMessage(Dart_CObject* root) {
 
   // Check that the two messages are the same.
   CompareDartCObjects(root, new_root);
+
+  free(buffer);
 }
 
 
@@ -147,6 +149,7 @@ static void ExpectEncodeFail(Dart_CObject* root) {
   ApiMessageWriter writer(&buffer, &malloc_allocator);
   const bool result = writer.WriteCMessage(root);
   EXPECT_EQ(false, result);
+  free(buffer);
 }
 
 
@@ -1653,6 +1656,7 @@ UNIT_TEST_CASE(MismatchedSnapshotKinds) {
     EXPECT(isolate == NULL);
     EXPECT(error != NULL);
     EXPECT_SUBSTRING("got 'script', expected 'core'", error);
+    free(error);
   }
 
   {