Extract portable AS to its own definition and polyfill; Try running flatten/ssa before default optimizations, see WebAssembly/binaryen#1331

portable-assembly.js

@@ -0,0 +1,29 @@
+var globalScope = typeof window !== "undefined" && window || typeof global !== "undefined" && global || self;
+
+globalScope["clz"] = Math.clz32;
+globalScope["abs"] = Math.abs;
+globalScope["max"] = Math.max;
+globalScope["min"] = Math.min;
+globalScope["ceil"] = Math.ceil;
+globalScope["floor"] = Math.floor;
+globalScope["select"] = function select(ifTrue, ifFalse, condition) { return condition ? ifTrue : ifFalse; };
+globalScope["sqrt"] = Math.sqrt;
+globalScope["trunc"] = Math.trunc;
+
+function UnreachableError() {
+  this.stack = new Error().stack;
+}
+UnreachableError.prototype = new Error;
+UnreachableError.prototype.name = "UnreachableError";
+UnreachableError.prototype.message = "unreachable";
+
+globalScope["unreachable"] = function unreachable() { throw new UnreachableError(); };
+
+function AssertionError() {
+  this.stack = new Error().stack;
+}
+AssertionError.prototype = new Error;
+AssertionError.prototype.name = "AssertionError";
+AssertionError.prototype.message = "assertion failed";
+
+globalScope["assert"] = function assert(isTrue) { if (!isTrue) throw new AssertionError(); };