fix: Don't kludge accessors in compiled mode (google#5591)

In compiled mode we don't need to add exports to the global Blockly
object because they'll already be there - and attempting to do so
causes problems when a project imports multiple separate copies of
Blockly (which it shouldn't, but many plugins do).

This is part of the fix for google/blockly-samples#902.

core/blockly.js

@@ -742,7 +742,24 @@ exports.zelos = zelos;
 // declareLegacyNamespace only copies normal data properties, not
 // accessors.  This can be removed once all remaining calls to
 // declareLegacyNamspace have been removed.
-if (globalThis.Blockly && typeof globalThis.Blockly === 'object') {
+//
+// This is only needed in uncompiled mode (see
+// google/blockly-samples#902); in compiled mode the exports object is
+// already the value of globalThis.Blockly.  Because
+// closure/goog/base.js is not included in the compiler input, we
+// can't use goog.global['COMPILED'] to check if we are running in
+// compiled mode.  Instead, use existence of globalThis.goog itself
+// for this purpose.
+//
+// Note that this code will still attempt to redefine accessors on a
+// previously-imported copy of the Blockly library if both are
+// imported in uncompiled mode.  This will fail with TypeError as the
+// accessors are nonconfigurable (which is good, as otherwise one
+// accessors on one copy would call get/set functions on the other
+// copy!)
+if (globalThis.goog && globalThis.Blockly &&
+    typeof globalThis.Blockly === 'object' &&
+    globalThis.Blockly !== exports) {
   const descriptors = Object.getOwnPropertyDescriptors(exports);
   const accessors = {};
   for (const key in descriptors) {