Implement ** exponentiation operator.

Parser and Baseline JIT.

js/src/frontend/Parser.cpp

@@ -5871,7 +5871,8 @@ static const JSOp ParseNodeKindToJSOp[] = {
     JSOP_SUB,
     JSOP_MUL,
     JSOP_DIV,
-    JSOP_MOD
+    JSOP_MOD,
+    JSOP_POW
 };
 
 static inline JSOp
@@ -5918,7 +5919,8 @@ static const int PrecedenceTable[] = {
     9, /* PNK_SUB */
     10, /* PNK_STAR */
     10, /* PNK_DIV */
-    10  /* PNK_MOD */
+    10, /* PNK_MOD */
+    11  /* PNK_POW */
 };
 
 static const int PRECEDENCE_CLASSES = 10;
@@ -5940,8 +5942,8 @@ template <typename ParseHandler>
 MOZ_ALWAYS_INLINE typename ParseHandler::Node
 Parser<ParseHandler>::orExpr1(InvokedPrediction invoked)
 {
-    // Shift-reduce parser for the left-associative binary operator part of
-    // the JS syntax.
+    // Shift-reduce parser for the binary operator part of the JS expression
+    // syntax.
 
     // Conceptually there's just one stack, a stack of pairs (lhs, op).
     // It's implemented using two separate arrays, though.
@@ -5975,10 +5977,9 @@ Parser<ParseHandler>::orExpr1(InvokedPrediction invoked)
         // stack, reduce. This combines nodes on the stack until we form the
         // actual lhs of pnk.
         //
-        // The >= in this condition works because all the operators in question
-        // are left-associative; if any were not, the case where two operators
-        // have equal precedence would need to be handled specially, and the
-        // stack would need to be a Vector.
+        // The >= in this condition works because it is appendOrCreateList's
+        // job to decide if the operator in question is left- or
+        // right-associative, and build the corresponding tree.
         while (depth > 0 && Precedence(kindStack[depth - 1]) >= Precedence(pnk)) {
             depth--;
             ParseNodeKind combiningPnk = kindStack[depth];
@@ -6176,6 +6177,7 @@ Parser<ParseHandler>::assignExpr(InvokedPrediction invoked)
       case TOK_MULASSIGN:    kind = PNK_MULASSIGN;    op = JSOP_MUL;    break;
       case TOK_DIVASSIGN:    kind = PNK_DIVASSIGN;    op = JSOP_DIV;    break;
       case TOK_MODASSIGN:    kind = PNK_MODASSIGN;    op = JSOP_MOD;    break;
+      case TOK_POWASSIGN:    kind = PNK_POWASSIGN;    op = JSOP_POW;    break;
 
       case TOK_ARROW: {
         tokenStream.seek(start);