misc/wasm: improve error message if javascript polyfills are required

wasm_exec.js expects that either "require" is available or that the
globals "crypto", "TextEncoder" and "TextDecoder" are already defined.
Report a better error message if this is not the case, suggesting the
use of a polyfill.

Updates #41482

Change-Id: I5473cae15c98ae42e39f5928245b7762e7a5a8bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/261357


Trust: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

misc/wasm/wasm_exec.js

@@ -102,7 +102,7 @@
 		}
 	}
 
-	if (!global.crypto) {
+	if (!global.crypto && global.require) {
 		const nodeCrypto = require("crypto");
 		global.crypto = {
 			getRandomValues(b) {
@@ -110,6 +110,9 @@
 			},
 		};
 	}
+	if (!global.crypto) {
+		throw new Error("global.crypto is not available, polyfill required (getRandomValues only)");
+	}
 
 	if (!global.performance) {
 		global.performance = {
@@ -120,13 +123,19 @@
 		};
 	}
 
-	if (!global.TextEncoder) {
+	if (!global.TextEncoder && global.require) {
 		global.TextEncoder = require("util").TextEncoder;
 	}
+	if (!global.TextEncoder) {
+		throw new Error("global.TextEncoder is not available, polyfill required");
+	}
 
-	if (!global.TextDecoder) {
+	if (!global.TextDecoder && global.require) {
 		global.TextDecoder = require("util").TextDecoder;
 	}
+	if (!global.TextDecoder) {
+		throw new Error("global.TextDecoder is not available, polyfill required");
+	}
 
 	// End of polyfills for common API.