[wasm] Factor out WasmModuleInstance from ModuleEnv.

test/unittests/wasm/ast-decoder-unittest.cc

Index: test/unittests/wasm/ast-decoder-unittest.cc
diff --git a/test/unittests/wasm/ast-decoder-unittest.cc b/test/unittests/wasm/ast-decoder-unittest.cc
index 923c55460426079ffeebb68e13b2be036fa5a8f9..5538170a7baccf718225a198978f0ed1eaf9058e 100644
--- a/test/unittests/wasm/ast-decoder-unittest.cc
+++ b/test/unittests/wasm/ast-decoder-unittest.cc
@@ -1204,11 +1204,9 @@ namespace {
 class TestModuleEnv : public ModuleEnv {
  public:
   TestModuleEnv() {
-    mem_start = 0;
-    mem_end = 0;
+    instance = nullptr;
     module = &mod;
     linker = nullptr;
-    function_code = nullptr;
     mod.globals = new std::vector<WasmGlobal>;
     mod.signatures = new std::vector<FunctionSig*>;
     mod.functions = new std::vector<WasmFunction>;