Fix posting exceptions when a debugger is attached, fix UpdateDebugger, fully implement ThreadGroupReference.Children.

Posting exceptions and UpdateDebugger were broken by the recent stack-walking
rewrite.

dalvikvm never did ThreadGroupReference.Children correctly; it only admitted
that "system" is the parent of "main".

Change-Id: I386f2fa5e01fba56cb2a1af4f136dbf31da0ff07
diff --git a/src/thread.cc b/src/thread.cc
index 104b9b9..53942d9 100644
--- a/src/thread.cc
+++ b/src/thread.cc
@@ -1643,7 +1643,7 @@
         CHECK(reg_bitmap != NULL);
         const VmapTable vmap_table(m->GetVmapTableRaw());
         const DexFile::CodeItem* code_item = MethodHelper(m).GetCodeItem();
-        DCHECK(code_item != NULL);  // can't be NULL or how would we compile its instructions?
+        DCHECK(code_item != NULL) << PrettyMethod(m); // Can't be NULL or how would we compile its instructions?
         uint32_t core_spills = m->GetCoreSpillMask();
         uint32_t fp_spills = m->GetFpSpillMask();
         size_t frame_size = m->GetFrameSizeInBytes();