fix: favor mapping at 0th column (#120)

If the line reported at the 0th column is greater than the line reported at the calculated column, favor the line at the 0th column.

lib/source.js

@@ -171,20 +171,39 @@ function originalEndPositionFor (sourceMap, line, column) {
 }
 
 function originalPositionTryBoth (sourceMap, line, column) {
-  const original = sourceMap.originalPositionFor({
+  let original = sourceMap.originalPositionFor({
     line,
     column,
     bias: GREATEST_LOWER_BOUND
   })
   if (original.line === null) {
-    return sourceMap.originalPositionFor({
+    original = sourceMap.originalPositionFor({
       line,
       column,
       bias: LEAST_UPPER_BOUND
     })
-  } else {
-    return original
   }
+  // The source maps generated by https://github.com/istanbuljs/istanbuljs
+  // (using @babel/core 7.7.5) have behavior, such that a mapping
+  // mid-way through a line maps to an earlier line than a mapping
+  // at position 0. Using the line at positon 0 seems to provide better reports:
+  //
+  //     if (true) {
+  //        cov_y5divc6zu().b[1][0]++;
+  //        cov_y5divc6zu().s[3]++;
+  //        console.info('reachable');
+  //     }  else { ... }
+  //  ^  ^
+  // l5  l3
+  const min = sourceMap.originalPositionFor({
+    line,
+    column: 0,
+    bias: GREATEST_LOWER_BOUND
+  })
+  if (min.line > original.line) {
+    original = min
+  }
+  return original
 }
 
 function getShebangLength (source) {