Fix handling non-ES5 identifiers in JS

When a Kotlin identifiers contains non-ES5 chars, JS BE
either reports error (for published declarations) or mangles name
(for non-published ones). The old approach relied on wrong
assuption that Java identifier = ES identifier.
However, that's not true. This commit introduces functions that
check identifiers according to ES5.1 spec rather than
using Character.isIdentifierStart[Part]

See KT-17476

js/js.frontend/src/org/jetbrains/kotlin/js/naming/NameSuggestion.kt

@@ -294,7 +294,7 @@ class NameSuggestion {
                     // Make all public declarations stable
                     effectiveVisibility == Visibilities.PUBLIC -> mangledAndStable()
 
-                    descriptor is CallableMemberDescriptor && descriptor.isOverridableOrOverrides -> mangledAndStable()
+                    descriptor.isOverridableOrOverrides -> mangledAndStable()
 
                     // Make all protected declarations of non-final public classes stable
                     effectiveVisibility == Visibilities.PROTECTED &&
@@ -346,8 +346,28 @@ class NameSuggestion {
         @JvmStatic fun sanitizeName(name: String): String {
             if (name.isEmpty()) return "_"
 
-            val first = name.first().let { if (java.lang.Character.isJavaIdentifierStart(it)) it else '_' }
-            return first.toString() + name.drop(1).map { if (java.lang.Character.isJavaIdentifierPart(it)) it else '_' }.joinToString("")
+            val first = name.first().let { if (it.isES5IdentifierStart()) it else '_' }
+            return first.toString() + name.drop(1).map { if (it.isES5IdentifierPart()) it else '_' }.joinToString("")
         }
+
+        // See ES 5.1 spec: https://www.ecma-international.org/ecma-262/5.1/#sec-7.6
+        private fun Char.isES5IdentifierStart() =
+                Character.isLetter(this) ||   // Lu | Ll | Lt | Lm | Lo
+                Character.getType(this).toByte() == Character.LETTER_NUMBER ||
+                    // Nl which is missing in Character.isLetter, but present in UnicodeLetter in spec
+                this == '_' ||
+                this == '$'
+
+        private fun Char.isES5IdentifierPart() =
+                isES5IdentifierStart() ||
+                when (Character.getType(this).toByte()) {
+                    Character.NON_SPACING_MARK,
+                    Character.COMBINING_SPACING_MARK,
+                    Character.DECIMAL_DIGIT_NUMBER,
+                    Character.CONNECTOR_PUNCTUATION -> true
+                    else -> false
+                } ||
+                this == '\u200C' ||   // Zero-width non-joiner
+                this == '\u200D'      // Zero-width joiner
     }
 }

js/js.translator/testData/box/jsName/peculiarIdentifiers.kt

@@ -23,9 +23,9 @@ fun test3(): String {
 }
 
 fun test4(): String {
-    val `()` = "OK"
-    fun `[]`() = `()`
-    return `[]`()
+    val `1(¢)` = "OK"
+    fun `[£]`() = `1(¢)`
+    return `[£]`()
 }
 
 fun box(): String {