[wasm] Add support for import section.

src/wasm/ast-decoder.h

Index: src/wasm/ast-decoder.h
diff --git a/src/wasm/ast-decoder.h b/src/wasm/ast-decoder.h
index 957e678f2cf9d9cdae04223e2fc01076034c6473..465bacaab84918b21a1db437b37353940bfb25bc 100644
--- a/src/wasm/ast-decoder.h
+++ b/src/wasm/ast-decoder.h
@@ -132,6 +132,16 @@ struct FunctionIndexOperand {
   }
 };
 
+struct ImportIndexOperand {
+  uint32_t index;
+  FunctionSig* sig;
+  int length;
+  inline ImportIndexOperand(Decoder* decoder, const byte* pc) {
+    index = decoder->checked_read_u32v(pc, 1, &length, "import index");
+    sig = nullptr;
+  }
+};
+
 struct TableSwitchOperand {
   uint32_t case_count;
   uint32_t table_count;