Fix an off-by-one error in the stack frame iteration.

runtime/vm/code_descriptors_test.cc

Index: runtime/vm/code_descriptors_test.cc
diff --git a/runtime/vm/code_descriptors_test.cc b/runtime/vm/code_descriptors_test.cc
index 5c5e860109150ab4e4c704ac437e2d2bb3f1dc13..9361cb43ad46f402e4e476a46f5e03262110a56b 100644
--- a/runtime/vm/code_descriptors_test.cc
+++ b/runtime/vm/code_descriptors_test.cc
@@ -192,8 +192,8 @@ TEST_CASE(StackmapGC) {
   EXPECT(CompilerTest::TestCompileFunction(function_foo));
   EXPECT(function_foo.HasCode());
 
-  // Build and setup a stackmap for function 'A.foo' in order to test the
-  // traversal of stack maps when a GC happens.
+  // Build and setup a stackmap for the call to 'func' in 'A.foo' in order
+  // to test the traversal of stack maps when a GC happens.
   StackmapBuilder* builder = new StackmapBuilder();
   EXPECT(builder != NULL);
   builder->SetSlotAsValue(0);  // var i.
@@ -201,15 +201,22 @@ TEST_CASE(StackmapGC) {
   builder->SetSlotAsValue(2);  // var k.
   builder->SetSlotAsObject(3);  // var s2.
   builder->SetSlotAsObject(4);  // var s3.
+  builder->SetSlotAsObject(5);  // First argument to func(i, k).
+  builder->SetSlotAsObject(6);  // Second argument to func(i, k).
   const Code& code = Code::Handle(function_foo.unoptimized_code());
   // Search for the pc of the call to 'func'.
   const PcDescriptors& descriptors =
       PcDescriptors::Handle(code.pc_descriptors());
+  int call_count = 0;
   for (int i = 0; i < descriptors.Length(); ++i) {
     if (descriptors.DescriptorKind(i) == PcDescriptors::kFuncCall) {
       builder->AddEntry(descriptors.PC(i) - code.EntryPoint());
+      ++call_count;
     }
   }
+  // We can't easily check that we put the stackmap at the correct pc, but
+  // we did if there was exactly one call seen.
+  EXPECT(call_count == 1);
   const Array& stack_maps = Array::Handle(builder->FinalizeStackmaps(code));
   code.set_stackmaps(stack_maps);