Restrict the kinds of expressions that force a .concat call split

This avoids creating extra .concat calls unless the result could be
potentially observably different from real template evaluation.

packages/babel-plugin-transform-es2015-template-literals/src/index.js

@@ -2,7 +2,13 @@ export default function({ types: t }) {
   function buildConcatCallExressions(items) {
     let avail = true;
     return items.reduce(function(left, right) {
-      let canBeInserted = t.isLiteral(right);
+      // these can potentially observe whether the previous template expression
+      // has been evaluated by ToString or not
+      let canBeInserted = !(
+        t.isSequenceExpression(right) ||
+        t.isCallExpression(right) ||
+        t.isMemberExpression(right)
+      );
 
       if (!canBeInserted && avail) {
         canBeInserted = true;

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/escape-quotes/expected.js

@@ -1 +1 @@
-var t = "'".concat(foo, "' \"").concat(bar, "\"");
+var t = "'".concat(foo, "' \"", bar, "\"");

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/functions/expected.js

@@ -1 +1 @@
-var foo = "test ".concat(_.test(foo), " ").concat(bar);
+var foo = "test ".concat(_.test(foo), " ", bar);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/literals/expected.js

@@ -1 +1 @@
-var foo = "".concat(1, f, "oo", true).concat(b, "ar", 0).concat(baz);
+var foo = "".concat(1, f, "oo", true, b, "ar", 0, baz);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/multiple/expected.js

@@ -1 +1 @@
-var foo = "test ".concat(foo, " ").concat(bar);
+var foo = "test ".concat(foo, " ", bar);