Scala.js: Fully mangle names in the backend.

In dotty, `mangledString` does not mangle operators that are not at
the end of an identifier, nor characters that are not
JavaIdentifierPart's. This is OK for the JVM, but not the Scala.js
IR. This commit forces full encoding of all characters.

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSEncoding.scala

@@ -212,7 +212,42 @@ object JSEncoding {
 
   private def encodeMemberNameInternal(sym: Symbol)(
       implicit ctx: Context): String = {
-    sym.name.toString.replace("_", "$und").replace("~", "$tilde")
+    fullyMangledString(sym.name)
+  }
+
+  /** Work around https://github.com/lampepfl/dotty/issues/5936 by bridging
+   *  most (all?) of the gap in encoding so that Dotty.js artifacts are
+   *  compatible with the restrictions on valid IR identifier names.
+   */
+  private def fullyMangledString(name: Name): String = {
+    val base = name.mangledString
+    val len = base.length
+
+    // slow path
+    def encodeFurther(): String = {
+      val result = new java.lang.StringBuilder()
+      var i = 0
+      while (i != len) {
+        val c = base.charAt(i)
+        if (c == '_')
+          result.append("$und")
+        else if (Character.isJavaIdentifierPart(c))
+          result.append(c)
+        else
+          result.append("$u%04x".format(c.toInt))
+        i += 1
+      }
+      result.toString()
+    }
+
+    var i = 0
+    while (i != len) {
+      val c = base.charAt(i)
+      if (c == '_' || !Character.isJavaIdentifierPart(c))
+        return encodeFurther()
+      i += 1
+    }
+    base
   }
 
   def toIRType(tp: Type)(implicit ctx: Context): jstpe.Type = {