Fix yet another bug in "if+ expr" comprehensions

(that is, comprehensions without a "for" clause)

The comprehension would return the result infinitely instead of only
once.

Still part of #3975.

compiler-js/src/main/java/com/redhat/ceylon/compiler/js/ComprehensionGenerator.java

@@ -71,10 +71,21 @@ void generateComprehension(Comprehension that) {
  tail = "return function(){return " + finished + ";}";
  }
  } else if (startClause instanceof ExpressionComprehensionClause) {
- // return the expression result
- gen.out("return function(){return ");
+ // record exhaustion state: return a function that
+ // * on first call, returns the expression,
+ // * on subsequent calls, returns finished.
+ String exhaustionVarName = names.createTempVariable("exhausted"); 
+ gen.out("var ", exhaustionVarName, "=false");
+ gen.endLine(true);
+ gen.out("return function()");
+ gen.beginBlock();
+ gen.out("if(", exhaustionVarName, ") return ", finished);
+ gen.endLine(true);
+ gen.out(exhaustionVarName, "=true");
+ gen.endLine(true);
+ gen.out("return ");
  ((ExpressionComprehensionClause)startClause).getExpression().visit(gen);
- gen.out("};");
+ gen.endBlockNewLine(true);
  for (int i = 0; i < initialIfClauses; i++) {
  gen.endBlock();
  }