feat(Escaped Identifiers): add ability to use any symbol wrapped in back ticks.

js/js.ast/src/org/jetbrains/kotlin/js/backend/JsToStringGenerationVisitor.java

@@ -8,6 +8,7 @@ import org.jetbrains.kotlin.js.backend.ast.*;
 import org.jetbrains.kotlin.js.backend.ast.JsDoubleLiteral;
 import org.jetbrains.kotlin.js.backend.ast.JsIntLiteral;
 import org.jetbrains.kotlin.js.backend.ast.JsVars.JsVar;
+import org.jetbrains.kotlin.js.common.IdentifierPolicyKt;
 import org.jetbrains.kotlin.js.util.TextOutput;
 import gnu.trove.THashSet;
 import org.jetbrains.annotations.NotNull;
@@ -895,7 +896,19 @@ public class JsToStringGenerationVisitor extends JsVisitor {
                 p.print(((JsNameRef) labelExpr).getIdent());
             }
             else if (labelExpr instanceof JsStringLiteral) {
-                p.print(((JsStringLiteral) labelExpr).getValue());
+                JsStringLiteral stringLiteral = (JsStringLiteral) labelExpr;
+                String value = stringLiteral.getValue();
+                boolean isValidIdentifier = IdentifierPolicyKt.isValidES5Identifier(value);
+
+                if (!isValidIdentifier)  {
+                    p.print('\'');
+                }
+
+                p.print(value);
+
+                if (!isValidIdentifier)  {
+                    p.print('\'');
+                }
             }
             else {
                 accept(labelExpr);

js/js.ast/src/org/jetbrains/kotlin/js/common/IdentifierPolicy.kt

@@ -0,0 +1,52 @@
+package org.jetbrains.kotlin.js.common
+
+private fun Char.isAllowedLatinLetterOrSpecial(): Boolean {
+    return this in 'a'..'z' || this in 'A'..'Z' || this == '_' || this == '$'
+}
+
+private fun Char.isAllowedSimpleDigit() =
+    this in '0'..'9'
+
+private fun Char.isNotAllowedSimpleCharacter() = when (this) {
+    ' ', '<', '>', '-', '?' -> true
+    else -> false
+}
+
+fun Char.isES5IdentifierStart(): Boolean {
+    if (isAllowedLatinLetterOrSpecial()) return true
+    if (isNotAllowedSimpleCharacter()) return false
+
+    return isES5IdentifierStartFull()
+}
+
+// See ES 5.1 spec: https://www.ecma-international.org/ecma-262/5.1/#sec-7.6
+private fun Char.isES5IdentifierStartFull() =
+    Character.isLetter(this) ||   // Lu | Ll | Lt | Lm | Lo
+            // Nl which is missing in Character.isLetter, but present in UnicodeLetter in spec
+            Character.getType(this).toByte() == Character.LETTER_NUMBER
+
+
+fun Char.isES5IdentifierPart(): Boolean {
+    if (isAllowedLatinLetterOrSpecial()) return true
+    if (isAllowedSimpleDigit()) return true
+    if (isNotAllowedSimpleCharacter()) return false
+
+    return isES5IdentifierStartFull() ||
+            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
+}
+
+fun String.isValidES5Identifier(): Boolean {
+    if (isEmpty() || !this[0].isES5IdentifierStart()) return false
+    for (idx in 1 until length) {
+        if (!get(idx).isES5IdentifierPart()) return false
+    }
+    return true
+}